Identification of an abdominal ganglion neuron antagonizing L7-driven gillcontraction in Aplysia

Gill motor neuron L7-induced longitudinal shortening of the gill in Aplysia kurodai and A. juliana was suppressed when extracellular stimuli were appl ied to a restricted dorsal central region of the abdominal ganglion. We fou nd a neuron there which antagonized the L7-driven contraction. Since the co ntraction was suppressed when the identified neuron was activated simultane ously with L7, we refer to the newly found neuron as "Anti-L7". Anti-L7 did not change the L7 impulse generation in the abdominal ganglion. No direct synaptic connection from L7 to Anti-L7 was detected. A fluorescent dye inje cted into the soma of Anti-L7 revealed that the neuron sent axonal branches to the branchial nerve. These results may show that Anti-L7 antagonizes L7 at the periphery inside the gill, rather than in the abdominal ganglion. E JPs induced by L7 were unaffected by Anti-L7. Activation of Anti-L7 alone d id not induce any change in tone or membrane potential of the gill musculat ure. The suppressive effect of Anti-L7 lasts many seconds after the cessati on of a train of Anti-L7 impulses. The results may suggest that the suppres sion is mediated through an inhibitory neuromodulatory mechanism without in hibition of L7 itself.